1. Field of the Invention
The present invention relates to the field of noise shaped digital amplifiers which directly convert digital signals to a power output. More specifically, the present invention relates to such digital amplifiers which are powered by power supplies having ripple and noise, and which include circuitry to compensate for the power supply voltage variations.
2. Description of Prior Art
Delta sigma modulation has become the standard technique for converting audio signals from the digital domain to the analog domain. For a good overview of the art, xe2x80x9cDelta Sigma Data Converters, by Norsworthy, Schreier and Temes (IEEE Press, 1997) is recommended. There is increasing interest in the use of noise shaping directly in power amplification to produce digital amplifiers. Very often this is accomplished by the combination of a delta sigma noise shaping element with a pulse width modulator, or PWM. For an overview of this technology, see U.S. Pat. Nos. 5,784,017 and 5,548,286 by Craven, U.S. Pat. No. 5,815,102 by the present inventor (incorporated herein by reference), U.S. patent application Ser. No. 09/163,235 by the present inventor (incorporated herein by reference), and International Patent Application No. PCT/DK97/00133 by Risbo. One of the significant difficulties in the production of such a system is the need for a well regulated power supply (or supplies). A potential solution to this problem is identified in U.S. Pat. No. 5,559,467, by Smedly. Smedly correctly identifies the need to use the value of the power supply voltage to modify the operation of the modulation, but proposes a solution that creates its own distortion. In addition, in the case of more that one power supply, it is necessary to use the value of both supplies in calculating the proper output.
With regard to the Smedly design, the first source of distortion is the memory, or state, in the noise shaping converter. The value of these state variables is referenced to the prior supply voltage, but the feedback will be used to modulate a future voltage. The noise cancellation which normally occurs cannot properly be canceled, as would happen in the normal, theoretic noise shaping case. In addition, referring to FIG. 5 of Smedly, the voltage across capacitor 36 is not the same as that across capacitor 38. This will induce other kinds of distortion. This second type of distortion would not occur in a four switch full bridge configuration, but would be significant in the structure drawn in FIG. 5.
A need remains in the art for a digital amplifier that properly compensates for the lack of regulation in its power supplies, without introducing any new sources of distortion.
It is an object of the present invention to provide a digital amplifier that properly compensates for the lack of regulation in its power supplies, without introducing any new sources of distortion.
As used herein, the term xe2x80x9cdigital amplifierxe2x80x9d applies to an amplifier which directly converts to a power output. The digital amplifier of the present invention comprises a delta sigma noise shaper feeding a pulse wave modulator (PWM) which drives a load such as a speaker. The delta sigma converter includes circuitry to correct for the nonlinear effects of pulse wave modulation. The delta sigma converter further includes circuitry to use a digital representation of the voltage coming out of the power supplies in the circuit to correct the pulse width output to compensate for the varying power supply voltages.
A digital amplifier according to the present invention comprises a delta sigma modulator having a direct signal path and a feedback signal path, with the audio signal as its input, and supplying a noise shaped signal as its output, and an output stage for converting the noise shaped signal into a power output. The output stage includes a power supply for supplying at least one level of voltage and power output circuitry powered by the power supply for generating a digital output signal according to the noise shaped signal. Compensating circuitry for correcting for variations in the voltage level supplied by the power supply includes means for measuring the voltage level of the power supply and means for adjusting the delta sigma modulator feedback by applying a function to the feedback according to the measured output voltage.
In the preferred embodiment, the compensating circuitry also modifies the direct path of the delta sigma modulator by applying a function which is substantially the inverse of the function applied by the feedback adjusting means. The direct path may be modified by applying a constant scalar to the direct path, or by continuously modifying the direct path according to the inverse of the effective output voltage.
The digital amplifier output stage preferably includes a pulse wave modulator for converting the noise shaped signal into a signal having various pulse widths related to the level of the noise shaped signal. Alternatively, the output stage could use a class D stage.
Generally, the power supply block supplies two levels of voltage, the voltage measuring means measures the two levels of voltage, and the compensating circuitry adjusts the feedback path and modifies the direct path according to the measured voltages.
As a feature, where the delta sigma modulator includes at least two integrator stages, the feedback applied to one stage is nontrivially different from the feedback applied to another stage, in order to correct for distortion introduced after the noise shaper.
The invention is an improvement in digital to analog conversion where the conversion is noise shaped, and the final output is created by switches connected to an unregulated power supply. No division circuit is utilized in the input signal path, as this would cause the above mentioned distortion. Instead, the operation of the noise shaper is modified to correctly reflect the output values being represented. Specifically, the quantizer and its feedback must be modified. In addition, two analog to converters (ADCs) are used in the case of two power supplies.